A voltage source PWM inverter enables stepless speed and torque control of AC motors, allowing reduced energy consumption and increased control performance. The use of a PWM inverter, however, not only brings advantages but also causes unwanted effects in the motor. The output voltage of the inverter consists of sharp-edged voltage pulses, producing bearing currents and high voltage stresses in motor insulations [1], [2]. The oscillation at the switching frequency causes additional losses and acoustic noise. These phenomena can be eliminated by adding an LC filter to the output of the PWM inverter. In addition, the EMI shielding of the motor cable may be avoided if the voltage is nearly sinusoidal.
Adding an LC filter to a variable speed drive makes the motor control more difficult. Usually, a simple volts-per-hertz control method is chosen. Better control performance is achieved by using vector control, i.e. field oriented control. However, there are only few publications that deal with the vector control of a motor fed through an LC filter [3]–[5]. In these papers, an extra current or voltage measurement was necessary, and a speed encoder was used. In order to obtain cost savings and reliability improvements, a full-order observer was proposed in [6], making additional current or voltage measurements unnecessary.
Recently, the speed sensorless control of ac motors has become popular. Promising estimation methods for speed sensorless induction motor drives are speed-adaptive full-order observers [7] combined with improvements in regeneration mode operation [8], [9]. However, a speed sensorless control methods for induction motor supplied through an LC filter are not yet published.
A problem associated with the prior art control systems with an output filter is the need for either the measurements of current or voltage from the motor or the use of speed encoders. Both of the above solutions can increase costs both for installation and maintenance on the system.